This invention relates to an impedance converting device and, more particularly, to an impedance converting device having a tri-plate structure.
Impedance converting devices which comprise a microstrip line are generally known. An impedance converting device having a microstrip line will be called a first impedance converting device.
The first impedance converting device comprises a grounded conductor member and a dielectric member located on the grounded conductor member. A microstrip line is formed on the dielectric member as a line conductor member which allows a signal to pass therethrough. The microstrip line has an open stub line including an impedance converting section having an inductive reactance element and a capacitive reactance element. Furthermore, at least one metal portion may be attached to the microstrip line in order to adjust the reactance of the impedance converting section.
However, an electromagnetic wave is inevitably irradiated from the microstrip line to outside of the impedance converting device on the basis of a high frequency signal passing through the microstrip line since nothing is located above the microstrip line in the first impedance converting device. In addition, an electromagnetic wave may be applied from outside of the impedance converting device to the microstrip line. In order to shield the electromagnetic wave, it is necessary to cover the first impedance converting device with a metal cover. As a result, the first impedance converting device becomes large. Furthermore, the metal cover affects the high frequency characteristics of the microstrip line.
On the other hand, an impedance converting device is known which comprises a strip line. This impedance converting device having a strip line will be called a second impedance converting device.
The second impedance converting device is disclosed in Japanese Unexamined Publication Tokkai Sho 62-268202 (268202/1987). The second impedance converting device comprises a tri-plate structure having first and second grounded conductor members and a strip line which is located in a space formed between the first and the second grounded conductor members. The strip line may be called the line conductor member in the second impedance converting device. The strip line is supported by a dielectric member inserted in the space. The strip line comprises an impedance converting section having an inductive reactance element and a capacitive reactance element.
Since the strip line is located in the space formed by the first and the second grounded conductor members, it is possible for the second impedance converting device to shield an electromagnetic wave. In other words, the second impedance converting device has an electromagnetic shielding effect.
However, it is difficult to vary the reactance of the impedance converting section in the second impedance converting device because the strip line is located in the space formed by the first and the second grounded conductor members. Thus, it is difficult to vary the impedance converting characteristics of the second impedance converting device.